This invention relates generally to solar arrays and, more particularly, to flexible, lightweight solar arrays for use on spacecraft. The use of solar arrays to provide electrical power to satellites and other spacecraft poses a number of related electrical, mechanical, and thermal problems.
In general, a solar array is continuously pointed toward the sun. In this orientation, the rear face of the array, facing away from the sun, accumulates an electrical charge from the space plasma, which eventually exceeds the breakdown threshold between the array and the spacecraft "ground." Unless protective means are provided, the build-up of charge on the rear face of the array will cause periodic arching discharges between the array and the spacecraft ground. The breakdown voltage may be as high as 10 kv (kilovolts), and the occurrence of arcing discharges is undesirable since the discharges may affect the operation and reliability of the array and the equipment it powers.
Coating the rear face of the array with a grounded metal layer solves this problem, but raises another one in that the metal layer can degrade the thermal performance of the array. Since the array receives solar radiation on its front face, and converts only a small fraction of the radiation to electrical power, the array must be capable of re-radiating a large amount of energy from its rear face.
Grounding layers have been used on relatively rigid solar arrays and on lightweight blanket-type arrays. A rigid array with a conductive paint layer on the rear face is described in "TDRSS Solar Array Design Guidelines for Immunity to Geomagnetic Substorm Charging Effects," by G. T. Inouye and J. M. Sellen, Jr., Proc. Thirteenth IEEE Photovoltai Specialists Conf., June 5-8, 1978. In the structure described in this paper, solar cells are supported on a rigid and grounded aluminum honeycomb substrate, which has on its rear face a layer of Kapton coated with conductive paint.
Lightweight solar arrays are typically supported on a flexible substrate of Kapton, a polyimide film material manufactured by the Du Pont de Nemours company. The Kapton substrate can be rolled or folded for transport into space, and later deployed into a flat sheet or other desired shape, supported on a rigid frame. It has been suggested in U.S. Pat. No. 4,043,834 issued in the name of Rusch, that the rear face of a Kapton substrate used in a solar array be coated with an adhesive impregnated with a conductive material. However, the patent makes no reference to possible thermal problems.
Another suggestion of the prior art is to be found in UK Patent Application No. 2 062 189 A, which describes a Kapton blanket structure in which a first conductive layer is sandwiched between two Kapton layers, and an optional second conductive layer is coated on the rear face of the substrate structure. Adhesive is the suggested form of the first conductive layer, and the optional rear-face layer is described as vacuum-deposited or painted.
UK Patent Application No. 2 100 510 A describes an approach in which an indium-tin-oxide (ITO) conductive layer is formed on both the front and rear faces of a solar array, and is further coated with a Kapton layer. Accordingly, there is no conductive layer on the rear face and no consideration is given to thermal problems.
It will be appreciated from the foregoing that there is still need for improvement in the field of solar arrays subject to space charge build-up and thermal problems. The present invention provides a simple but highly effective solution to these problems.